As one of fiber laser devices used in, for example, a processing machine, there has been known an MO-PA (Master Oscillator-Power Amplifier) fiber laser device which amplifies light, generated by a seed light source such as a laser oscillator (MO: Master Oscillator), by an amplifier (PA: Power Amplifier) including an amplification optical fiber and emits the light. Light having a wavelength band of visible light has been emitted from such a fiber laser device by converting light having a wavelength band of near infrared light to the short wavelength side by using a wavelength conversion element.
Upon such wavelength conversion, wavelength conversion tends to be less effective when light before the wavelength conversion includes a higher-order mode. Thus, it is preferable that light entering a wavelength conversion element include light of the fundamental mode only and do not include light of a higher-order mode as much as possible. In addition, when the light emitted from the amplification optical fiber is focused, it is preferable that light entering a wavelength conversion element include light of the fundamental mode only and do not include light of a higher-order mode as much as possible. On the other hand, there is a demand for using, as an optical fiber such as an amplification optical fiber, an optical fiber including a core having a diameter larger than a single mode fiber in order to allow light having high power to propagate associated with higher output of a fiber laser device. Even in such a case, there is a demand for emitting light with a good beam quality, which includes light of the fundamental mode and reduced light of higher-order modes.
Patent Document 1 listed below describes an example of an amplification optical fiber including a core selectively doped with an active element in order to mainly amplify light of an LP01 mode and less amplify light of an LP11 mode comparing to the light of the LP01 mode. Such an amplification optical fiber can amplify light of the fundamental mode and suppress amplification of light of the LP11 mode at the same time. Therefore, light including light of the fundamental mode and reduced light of the LP11 mode can be emitted.